>>I'd say it means that tdNullable can be null http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx#Y1537
>>
>>You may want to test tdNullable.HasValue to verify whether it contains a value
>
>Is there a way to avoid this question mark if there is a possiblity to receive a null value for the parameter?

I believe no. If the date can be null, then you need ? in the parameter definition.